Zinc oxide is a wide-bandgap semiconductor that is transparent to visible light. U.S. Pat. No. 4,751,149 to Vijayakumar et al. disclosed that the electrical conductivity of zinc oxide can be increased greatly by adding small amounts of the group 13 elements boron, aluminum, gallium or indium. Addition of these dopants, however, decreases the transparency of the zinc oxide. U.S. Pat. No. 4,990,286 to Gordon disclosed that fluorine is also effective in increasing the electrical conductivity of zinc oxide films, and that the resulting films have higher transparency than those doped with group 13 dopants. The greater transparency of the fluorine-doped films gives them better performance in devices such as solar cells, flat-panel display devices, electrochromic absorbers and reflectors, energy-conserving heat mirrors, and anti-static coatings.
Zinc oxide may be formed by reactions of vapors of a wide variety of zinc-containing compounds. These processes are called chemical vapor deposition (CVD) processes.
Some widely-used zinc CVD precursors are dialkylzinc compounds, having the general form ZnR.sub.2, in which R stands for an organic radical, such as methyl, ethyl, isopropyl, etc. Examples of these precursors are dimethylzinc and diethylzinc. In U.S. Pat. No. 4,751,149 vapors of a dialkylzinc compound are mixed with water vapor and diborane near a heated surface, on which a layer of boron-doped zinc oxide is deposited. In U.S. Pat. No. 4,990,286 vapors of a dialkylzinc compound are mixed with alcohol vapor and hexafluoropropene near a heated surface, on which a layer of fluorine-doped zinc oxide is deposited.
Dialkylzinc compounds have some disadvantages in a CVD process. Dialkylzinc compounds are pyrophoric, that is they ignite spontaneously in air, so that they are a serious fire hazard. Even small amounts of oxygen in the CVD chamber cause powdered zinc oxide to form. Thus the prior art CVD processes for forming zinc oxide films from dialkylzinc compounds can be disrupted by air leaks into the CVD chamber.
Complexes of dialkylzinc compounds with tetrahydrofuran have been proposed as CVD sources for zinc oxide by T. Kaufmann, G. Fuchs and M. Webert, in Crystal Research & Technology, vol. 23, pp. 635-639 (1988). However, these complexes are not very stable, and they dissociate in the gas phase unless a more than 100-fold excess of tetrahydrofuran is used.
Non-pyrophoric zinc CVD sources have also been found. Zinc propionate was discovered to be a CVD source material for zinc oxide films by Y. A. Savitskaya and L. A. Ryobova, Zhurnal Prikladnoi Khimii, Vol. 37, pp. 796-800 (1962). Zinc acetylacetonate was used similarly by L. A. Ryabova, Y. S. Savitskaya and R. N. Sheftal, Izvestia Akademii Nauk. SSSR, Neorganischeskie Materialy, Vol. 4, p. 602 (1968). Zinc acetate was claimed as a CVD precursor for films with electronic purposes by M. Hattori and T. Maeda in Japan. Kokai 73 29,699 (1973). These non-pyrophoric sources are solids, so they are less convenient to handle than liquids are, and they are more difficult to vaporize.